The rotation number detector that outputs a signal according to the number of rotations of the rotation shaft of a turbo charger detects a movement of the blade provided on the rotation shaft of the turbo charger. For example, the rotation number detector disclosed in a patent document, Japanese Patent Laid-Open No. 2013-224847 A (patent document 1) detects the number of rotations of the rotation shaft, a vibration of the rotation shaft, and the clearance between the blade and a sensor by processing a signal outputted from the sensor disposed in a non-contact manner relative to the blade. Among these detection items, the number of rotations of the rotation shaft is detected (i) by dividing the output signal of the sensor with a dividing ratio, which is the same number as the number of the blades of the turbo charger, and (ii) by detecting a cycle of the divided signal.
However, the dividing ratio of the rotation number detector in the patent document 1 for dividing the output signal from the sensor is fixed. Therefore, when the number of rotations of the rotation shaft of the turbo charger changes in a wide range (e.g., from 10,000 rotations to 300,000 rotations), a detection time to detect the length of the cycle of the divided signal also changes in a wide range (i.e., 30 times in the above example). On the other hand, the control cycle time for controlling the electronic control unit that receives the signal outputted from the rotation number detector is usually configured on an order of milliseconds. Therefore, in case that the output signal of the rotation number detector having a short cycle is adapted to the control cycle time of the electronic control unit, the output signal from the rotation number detector at a time when the turbo charger is rotating at a very low rotation number may be updated too slowly in terms of the update cycle of the electronic control unit, thereby making it difficult to be adapted to the control cycle time of the electronic control unit.
Further, in case that the long cycle output signal is divided by a very high dividing ratio to have a divided cycle on an order of milliseconds, the shorter cycle output signal at a time of a very high speed rotation of the rotation shaft of the turbo charger may become too short for an accurate cycle measurement, if the dividing ratio is fixed to a certain number. That is, the detection accuracy of the cycle measurement may be deteriorated in such detection scheme.